How far in advance can I apply for a yearly marriage extension in Phuket before it expires?

Ahh I see

@David *****

you're not asking how far in advance, you're asking how close to the expiration date you can apply 🙂

There is no minimum required time left on your current stamp to apply for a new year extension

You can apply for a new yearly extension right up until the last day your current stamp is valid for. Obviously don't wait until the last day because if you're missing a document or something you run on overstay trying to sort it out but no issue at all applying a 2 weeks, 1 week, a few days before..